The present disclosure relates to compensate hydrogen induced measurement error of fiber Bragg grating sensors. More particularly, it relates to correlate the hydrogen induced Bragg wavelength shift with hydrogen induced fiber transmission loss and further actively compensate Bragg wavelength shift.
Fiber optic sensors are attractive for harsh environment applications due to advantages including good high-temperature capability, corrosion resistance and electromagnetic insensitivity. The oil and gas industry has increasingly adopted fiber optic sensors to monitor producing zones and take actions to optimize production. Fiber Bragg gratings (FBG) are found to be one of the fiber optic sensors suitable for oil/gas application because of ease of multiplexing and simple wavelength decoding.
FBG is a structure formed by periodically changing the refractive index of a fiber core. When a broadband light is launched into a fiber, only the light at a specific wavelength will be reflected, which is defined as Bragg wavelength and can be modulated by measurands of interest such as temperature, strain, chemicals etc. More than one FBG sensor can be serially multiplexed in a fiber as illustrated 120 in FIG. 1 and share the same interrogation system. The FBG sensors should have different Bragg wavelengths to allow them to be distinguished in the wavelength domain. The interrogation system includes a white light source or a wavelength tunable laser, several couplers and a spectrometer. Each FBG sensor can be used to measure a particular parameter of interest. For example, FBG sensor 1 could be used to measure temperature, and FBG sensor 2 could be used to measure strain. A reflective spectrum of two multiplexed FBG sensors is shown as 140.
One commonly adopted method to manufacture FBG is by illuminating a fiber with periodic interference pattern generated by ultra-violet (UV) light. The fiber used to make FBG's typically include photosensitive dopant, such as germanium and the refractive index modulation is believed to be formed by UV-induced breaking of electronic bonds in the Ge-based defects, releasing electrons which are thereafter re-trapped at other sites in the glass matrix.
It is known that the hydrogen diffusion into optical fibers is pervasive in the oil and gas well environment. The diffusion of hydrogen into the core of the Germanium-doped fiber will cause a change in the refractive index and consequently modify the Bragg wavelength of an FBG written into the core of the fiber. FIG. 2 shows the hydrogen induced Bragg wavelength shift over time. The hydrogen-induced change is found to be temperature dependent in short term: at low temperature (<150 C), the Bragg wavelength shift is mainly dominated by hydrogen diffusion and at elevated temperature (>150 C), the Bragg wavelength shift is caused by permanent reaction with hydrogen and diffusion of Hydrogen. In long term, the shift will level out over time to a value that is proportional to the hydrogen concentration and the defects in the fiber. In the wellbore, temperatures can be up to 300° C. and it means the FBG based sensor would have inevitable wavelength drift error over time. Thus, a need remained in the art for a technique to actively compensate the Bragg wavelength shift of FBG sensor used in hydrogen-rich, high temperature, harsh environment such as oil and gas wells. This disclosure provides this ability.